Clarifying and/or filtering of green liquor and an apparatus therefore

ABSTRACT

A method and apparatus for treating green liquor from a sulphate pulp mill enhances the clarification of the green liquor and may easily be integrated into existing pulp mills. A first portion of the green liquor from the sulphate pulp mill is clarified in a clarifier to produce a clarified green liquor and a dregs-containing green liquor. The dregs-containing green liquor is filtered in a green liquor filter to produce filtrate and dregs. The filtrate is combined) with the clarified green liquor to produce a first combined stream; and the dregs are filtered in a dregs filter. A second portion of green liquor from the sulphate pulp mill is combined with the dregs-containing green liquor to produce a second combined stream, and the second combined stream is introduced into the green liquor filter. The dregs from step (b) are washed prior to filtering the dregs, the green liquor filter and washer comprising a single structure.

CROSS REFERENCE TO RELATED APPLICATION

This application is a U.S. national phase of PCT/Fl95/00125,International Publication No. WO 95/25198.

CROSS REFERENCE TO RELATED APPLICATION

This application is a U.S. national phase of PCT/Fl95/00125,International Publication No. WO 95/25198.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a method of and an apparatus fortreating green liquor formed in a sulphate pulp mill, especially to amethod of and an apparatus for enhancing the clarifying of green liquor.

An essential part of the process of producing kraft pulp is chemicalrecovery. The chemical recovery process includes production of whiteliquor by causticizing, whereby lime milk and green liquor are allowedto react so as to produce lime mud and white liquor. Green liquor isproduced by directing a chemical melt containing sodium chemicals fromthe bottom of the furnace of a recovery boiler into a separatedissolving tank, in which the melt is dissolved into weak white liquor.The most important chemicals contained by the green liquor are sodiumcarbonate and sodium sulphide. Green liquor also contains insolublecompounds, such as oxides of metals, silicates, soot and otherimpurities. The insoluble impurities, dregs, are a light, fluffymaterial that must be removed from the green liquor. Otherwise thesecompounds will be concentrated in the chemical recovery loop which isdetrimental to the production of white liquor by causticizing.

Usually the dregs are separated from the green liquor by settling in aclarifier, which is a vertical cylinder with a spherical bottom. In themiddle of the clarifier is a smaller cylinder, into which the greenliquor is introduced and in which the flow is damped. The dregs settleon the bottom of the clarifier and are scraped from there into a pocketat the bottom of the clarifier and then introduced to a dregs filter, inwhich the dregs are washed with water for recovering alkali. Thealkaline washing liquid is used in the melt dissolver, while theclarified green liquor is directed to causticizing.

Usually the clarifier functions both as a storage tank and a dregsseparator. The bottom functions as a clarifier and the rest as a storagetank. Clarified liquor rises from the clarifier portion to the storageportion, from which clarified liquor is discharged as needed through aliquor discharge duct. Because of the fineness of the dregs to beseparated the separation process must be highly efficient. As the sizeof the mills increases and environmental regulations tighten, separationby clarifying conventionally leads to large equipment, with thediameters of clarifiers exceeding 30 meters. Additionally, a clarifyingprocess is always sensitive to disturbances, such as fluctuations inpulp production process, whereby even a correctly dimensioned equipmentyields green liquor with too much impurities.

So, in order to succeed, clarifying requires stability in both thequality and quantity of the introduced green liquor. An increase in thequantity of treated liquor would thereby necessitate an additionalclarifier. An alternative is to treat part of the green liquor byfiltering it with a filter parallel with the clarifier. Nevertheless, inthis case directing the dregs from both the clarifier and the additionalfilter to the dregs filter, in which the separated dregs are washed,could result in overloading the dregs filter of the mill.

The purpose of the present invention is to provide a method capable ofincreasing the capacity and/or operation capability of an existing greenliquor clarifier without sacrificing the quality of the clarified greenliquor. An enhanced operation capability can be useful, should the rateof settling of the dregs decrease for some reason. Further, anotherpurpose of the invention is to improve the operation of the dregs filterregardless of any increase in the quantity of green liquor.

Characterizing features of the method according to the invention forachieving the above-mentioned purposes are that

a) at least a portion of the green liquor stream of the mill is directedto a clarifier, from which clarified green liquor is discharged from thetop and dregs-containing green liquor from the bottom; and

b) dregs-containing green liquor from step a) is introduced into afilter, in which it is filtered for separating filtrate from dregs.

Characterizing features of the apparatus according to the invention arethat it comprises

a clarifier and a filter connected in series for introducingdregs-containing green liquor from the clarifier to the filter,

means for introducing green liquor into the filter,

means for discharging clarified green liquor from the clarifier and aduct for discharging filtered green liquor from the filter and

means for removing green liquor dregs from the filter.

The connection according to the invention enables a portion of the greenliquor stream to be introduced past the clarifier directly to the filteralong with the dregs-containing green liquor.

The method and apparatus according to the invention enhance theclarification of green liquor by connecting a filter in series with theclarifier, the filter further treating the dregs-containing green liquorfrom the clarifier. The dregs separated from the green liquor can, atthis stage, be also washed in this filter for recovering alkali.According to present practice the dregs are washed at the dregs filter.Thus, the present invention greatly improves and enhances the operationof the solids filter, as the alkali content of the dregs to be filteredthere is greatly reduced. Also, the filter produces a much moreconcentrated dregs than the clarifier. Usually, the solids concentrationof the dregs from a clarifier is about 2-3%, whereas the connectionaccording to the present invention can increase the concentration toabout 5-15%.

The method according to the invention is not as prone to variousdisturbances as clarification alone. Should the clarification be forsome reason disturbed, the volume of clarified green liquor from theclarifier can be reduced to include only the clearest green liquor. Thevolume of the stream of dregs-containing green liquor to the filter isthen accordingly increased. Thus, both the clarifier and the filterproduce green liquor with a low enough dregs content for further use.

The method of the invention is for treating green liquor of a sulphatepulp mill to enhance clarification thereof, and/or to enhanceutilization thereof. The method comprises the steps of: (a) clarifyingat least a portion of the green liquor from the sulphate pulp mill toproduce a clarified green liquor and a dregs-containing green liquor;(b) filtering the dregs-containing green liquor to produce filtrate anddregs; (c) combining the filtrate from step (b) with the clarified greenliquor from step (a) to produce a first combined stream; and (d) washingand then filtering, or filtering, the dregs from step (b). The filteringtakes place in a dregs filter. Also, the green liquor treated in step(a) may comprise a first portion of green liquor from the sulphate pulpmill and there may be a further step of combining a second portion ofgreen liquor from the sulphate pulp mill with the dregs-containing greenliquor from step (a) to produce a second combined stream, and using thesecond combined stream in the practice of step (b).

The filter connected in series with the clarifier can be any filtercapable of treating dregs-containing green liquor. Such filters include,e.g., drum, disk and tube filters with a filtering layer formed by limemud. Corresponding vacuum filters can also be used. A preferred filterfor this purpose is described in international patent applicationPCT/FI94/00485.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic illustration of exemplary apparatus forcarrying out an exemplary method according to the present invention.

DETAILED DESCRIPTION OF THE DRAWING

The green liquor to be treated is introduced into the clarifier 10 viaducts 12 and 13. Clarified green liquor with low enough dregs content isdischarged from the clarifier via conduit 14, while dregs-containinggreen liquor is directed from the bottom of the clarifier 10 to thefilter 18 via duct 16.

Because the dregs conduit 16 of many an old clarifier is underrated forthe volume of green liquor to be treated by the filter after an increasein the stream of liquor, a portion of the green liquor is directed pastthe clarifier to the filter 18 via duct 15.

In this embodiment the filter is an apparatus as described in theinternational patent application PCT/FI94/00485, the apparatuscomprising a plurality of filter elements having filter surfaces. Thegreen liquor to be treated is directed via ducts 16 and 17 to a liquiddistribution tray 22 located above the filter elements, in which thegreen liquor is divided to flow down on the filter surface of theelements. The film of liquid, flowing downwards, is filtered through thefilter surfaces of the elements so that the filtrate flows through thesurface while the solids remain on the surface. The filtrate, i.e.filtered green liquor, is discharged from the filter via duct 24 intotank 26, where gas 28 is separated from the filtrate. The gas can berecycled to the filter via a compressor 30 located along duct 32. Thefiltered green liquor is directed away from the tank 26 along duct 34and directed together with clarified green liquor via duct 36 to furthertreatment at the causticizing plant.

The green liquor from the filter can also be separately treated, i.e. itis not taken to the causticizing plant for production of white liquor.Filtered green liquor is usually cleaner than clarified green liquor, soit can be utilized in, e.g., the bleaching process. Sodium carbonate isseparated from the filtered green liquor and then further causticizedinto sodium hydroxide to be used in bleaching. Separation of sodiumcarbonate produces high-sulphidity green liquor to be preferably used inthe initial phase of pulp cooking.

Unfiltered dregs-containing green liquor is collected on the bottom ofthe filter, from where it is removed via duct 37 and redirected viasludge tank 38 (or directly from the filter) to the filtration surfacevia ducts 40 and 17. Dregs slurry is removed from the process via duct42, tank 44 and duct 46 to the dregs filter 48.

The load on the actual dregs filter 48 can be lightened by the methodaccording to the invention, as the invention provides for washing thedregs already in the filter 18. In this case dregs are intermittentlyfiltered in the filter 18. Instead of removing dregs slurry from thetank 38 during a filtration cycle, it is recycled and concentrated. Atthe end of the filtration cycle, the feed of green liquor to the filteris replaced by washing water from duct 50 and filtration is continued.Washing water, while mixing with the dregs, removes alkali therefrom.Subsequent to the washing, dregs are removed to the solids filter 48.Thus treated, dregs are cleaner and thicker than dregs that have beenconventionally directed from a clarifier directly to the dregs filter.

The efficiency of dregs washing can be improved by allowing it to form acake on the filtration surface. The filter 18 can also be a pressure orvacuum filter known per se, with either a drum or a disk as the filterelement.

The present invention is highly preferable in cases where the existingclarifier is either overloaded or its capacity must be increased. Theadvantages provided by the invention are listed in the following:

the clarifier can be operated at its rated capacity, whereby it producesclean liquor;

the space required by the new piece of machinery (the filter) is small;

all the dregs of the mill are treated in the filter, whereby theconcentration of the dregs is higher when it is directed for furthertreatment;

all the dregs can be pre-washed;

the actual dregs washing apparatus (dregs filter) is capable of treatingmore dregs than when collecting dregs from the bottom of the clarifier;

when increasing the production of the mill, the capacity of existingmachinery is often sufficient for final washing of dregs, even thoughthe amount of dregs increases.

We claim:
 1. A method of treating green liquor of a sulphate pulp millto enhance clarification thereof, comprising the steps of:(a) clarifyingat least a portion of the green liquor from the sulphate pulp mill toproduce a clarified green liquor and a dregs-containing green liquor;(b) filtering the dregs-containing green liquor to produce filtrate anddregs; (c) combining the filtrate from step (b) with the clarified greenliquor from step (a) to produce a first combined stream; and (d)filtering the dregs from step (b) in a dregs filter.
 2. A method asrecited in claim 1 wherein the green liquor treated in step (a)comprises a first portion of green liquor from the sulphate pulp mill;and comprising the further step of combining a second portion of greenliquor from the sulphate pulp mill with the dregs-containing greenliquor from step (a) to produce a second combined stream, and using thesecond combined stream in step (b).
 3. A method as recited in claim 2comprising the further step of (e) washing the dregs from step (b) priorto filtering the dregs in step (d).
 4. A method as recited in claim 3wherein steps (e) and (b) are practiced in a common filter.
 5. A methodas recited in claim 2 comprising the further step of treating the firstcombined stream to produce white liquor.
 6. A method as recited in claim3 comprising the further step of treating the first combined stream toproduce white liquor.
 7. A method as recited in claim 1 comprising thefurther step of (e) washing the dregs from step (b) prior to filteringthe dregs in step (d).
 8. A method as recited in claim 7 wherein steps(e) and (b) are practiced in a common filter.
 9. A method as recited inclaim 7 comprising the further step of treating the first combinedstream to produce white liquor.
 10. A method as recited in claim 1comprising the further step of treating the first combined stream toproduce white liquor.
 11. A method as recited in claim 1 wherein step(a) is practiced in an existing operating clarifier in a pulp mill; andcomprising the further step of adding a green liquor filter forpracticing step (b) to the sulphate pulp mill, and then operating thegreen liquor filter to practice step (b).
 12. A method of treating greenliquor of a sulphate pulp mill having a bleach plant, to enhanceutilization thereof, comprising the steps of:(a) clarifying at least aportion of the green liquor from the sulphate pulp mill to produce aclarified green liquor and a dregs-containing liquor; (b) filtering thedregs-containing liquor to produce filtrate and dregs; (c) using theclarified liquor from step (a) to produce white liquor; and (d)utilizing the filtrate from step (b) in the bleach plant.
 13. A methodas recited in claim 12 wherein step (d) is practiced by separatingsodium carbonate from the filtrate, and causticizing the filtrate intosodium hydroxide, and using the sodium hydroxide to facilitate bleachingin an alkaline bleaching stage in the bleach plant.
 14. A method asrecited in claim 13 comprising the further step (e) of using thefiltrate from which sodium carbonate has been separated as ahigh-sulphidity green liquor in an initial phase of pulp cooking in thepulp mill.
 15. A method as recited in claim 12 comprising the furtherstep (e) of washing the dregs from step (b).
 16. A method as recited inclaim 15 comprising the further step of filtering the dregs in a dregsfilter after the practice of step (e).
 17. A method of treating greenliquor of a sulphate pulp mill to enhance clarification thereof,comprising the steps of:(a) clarifying at least a portion of the greenliquor from the sulphate pulp mill to produce a clarified green liquorand a dregs-containing green liquor; (b) filtering the dregs-containinggreen liquor to produce filtrate and dregs; (c) combining the filtratefrom step (b) with the clarified green liquor from step (a) to produce afirst combined stream; and (d) washing the dregs from step (b).
 18. Amethod as recited in claim 17 wherein the green liquor treated in step(a) comprises a first portion of green liquor from the sulphate pulpmill; and comprising the further step of combining a second portion ofgreen liquor from the sulphate pulp mill with the dregs-containing greenliquor from step (a) to produce a second combined stream, and using thesecond combined stream in step (b).